1. Field of the Invention
The present invention relates generally to sealing gaskets used for pipe joints in which a male spigot pipe section is installed within a mating female socket pipe section.
2. Description of the Prior Art
Pipes formed from thermoplastic materials including polyethylene and PVC are used in a variety of industries. In forming a joint between sections of pipe, the spigot or male pipe end is inserted within the female or socket pipe end. An annular, elastomeric ring or gasket is typically seated within a groove formed in the socket end of the thermoplastic pipe. As the spigot is inserted within the socket, the gasket provides the major seal capacity for the joint. It is critical, during the installation process, that the gasket not be able to twist or flip since a displaced or dislocated gasket will adversely affect the ultimate sealing capacity of the joint.
Certain of the prior art techniques have involved the use of gaskets which relied entirely upon "lip" seals as opposed to a "compression seal." The commercially available prior art gaskets had a lip portion which was deformed by folding back as the male pipe section was inserted within the female pipe section. Lip seals generally rely upon the internal pipe pressure to provide a sealing effect. In some conditions, the prior art lip seals peeled away, allowing leakage through the joint. Particularly, under pulsating pressure conditions, commonly available lip seals can provide less than satisfactory results. A sudden decrease in internal pipe pressure can result in contamination of the sealing area and of the pipe interior.
One attempt to insure the integrity of the pipe joint involved the use of a pipe gasket having one region formed of an elastically yieldable sealing material, such as rubber, and a second distinct region formed of a more rigid material, such as a rigid plastic. See, e.g., U.S. Pat. No. 4,818,209, issued Apr. 9, 1989, to Peterson, et al. The rubber/plastic composite gasket required an adhesive interface between the gasket sections, was subject to degradation during belling operations or in the presence of heat and often lost sealability under low pressure conditions.
It is also critical to the success of the pipe joint that the sealing gasket be maintained in the proper position in its respective gasket groove during installation and use. One early attempt to ensure the integrity of pipe joints used under demanding conditions was to provide local reinforcement of the groove portion of the female socket end by means of a heavier wall thickness in this region of the pipe. In some cases, reinforcing sleeves were also utilized. Each of these solutions was less than ideal, in some cases failing to provide the needed joint integrity and often contributing to the complexity and expense of the manufacturing operation.
In the early 1970's, a new technology was developed by Rieber & Son of Bergen, Norway, referred to in the industry as the "Rieber Joint." The Rieber system employed a combined mould element and sealing ring for sealing a joint between the socket end and spigot end of two cooperating pipes formed from thermoplastic materials. In the Rieber process, the elastomeric gasket was inserted within an internal groove in the socket end of the female pipe as the female or belling end was simultaneously being formed. The provision of a prestressed and anchored elastomeric gasket during the belling process at the pipe factory provided an improved socket end for a pipe joint with a sealing gasket which would not twist or flip or otherwise allow impurities to enter the sealing zones of the joint, thus increasing the reliability of the joint and decreasing the risk of leaks or possible failure due to abrasion. The Rieber process is described in the following issued United States patents, among others: U.S. Pat. Nos. 4,120,521; 4,061,459; 4,030,872; 3,965,715; 3,929,958; 3,887,992; 3,884,612; and 3,776,682.
The Rieber system, with its integrally installed gasket provided a compression seal geometry, rather than relying upon the lip seal systems of the prior art. As the female socket end of the pipe cooled around the previously installed gasket, the elastomeric body of the gasket was compressed between a steel reinforcing ring and the groove formed in the socket end of the pipe. When the mating spigot was installed to make up a pipe joint, further compression of the gasket occurred in a radial direction along an internal compression region of the gasket.
Despite the advances offered by the Rieber process, the compression seal geometry presented problems in certain field applications. In some applications, particularly involving larger diameter pipe, the insertion force needed to install the male spigot end within the mating socket end required a high insertion force which could, on some occasions, cause the gasket to be distorted or displaced.
The present invention has, as one object, to provide a pipe gasket for use in such systems which offers the advantages of a lip seal without losing the advantages of a compression seal in a Rieber style gasket.
Another object of the invention is to provide such a pipe gasket which provides a minimally interfering trajectory and consequent lower insertion force for the male, spigot end when entering the female, spigot end to facilitate assembly of the pipe joint in the field.
Another object of the invention is to provide a sealing system of the above type in which a sealing gasket is installed within a groove provided within a female, socket end of a thermoplastic pipe in which the gasket is designed with an improved insertion geometry to provide a lower insertion force while maintaining a proper compression seal with respect to a mating male spigot end of pipe during low pressure conditions and while maintaining additional lip sealing during low pressure conditions.
Another object is to provide an improved gasket for use in such a sealing system in which the gasket is formed from a single, generally homogeneous composition, rather than having distinct regions of diverse materials.